This invention relates generally to methods and apparatus for CT imaging, and more particularly to methods and apparatus for reducing an x-ray dose to a patient during a computed tomography (CT) imaging scan.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In recent years, radiologists have become increasingly concerned about reducing the x-ray dose to a patient during a CT examination. In Europe, for example, strict guidelines have been instituted to prevent potential danger of exposing patients and operators to excessive x-ray radiation. New uses of CT imaging have increased the interest in limiting x-ray exposure of patients. In cardiac screening CT, for example, asymptomatic patients undergo routine CT scans to detect calcification in coronary arteries. In this and in other types screening examinations, x-ray dose to the patient should be kept to a minimum.
It would therefore be desirable to provide methods and apparatus for reducing an amount of x-ray dosage a patient receives during a CT imaging scan. More particularly, it would be desirable to minimize x-ray dosage to regions outside an organ of interest (OOI) whenever a relatively small organ, such as a heart, is the OOI being scanned.
There is therefore provided, in one embodiment, a method for scanning an region of interest of an object with a multi-slice computed tomography (CT) imaging system that reduces a radiation dose received by the object during a scan. The method includes steps of collimating the radiation beam of the CT imaging system into a fan-shaped radiation beam having at least a first region and a second region, the first region having a lesser angular extent than that of the second region, scanning an object having a region of interest (ROI) with the collimated radiation beam; and reconstructing an image of the object using the attenuation measurements collected during the scan, wherein the reconstruction utilizes attenuation measurements collected using the second region of the radiation beam to estimate projection data from portions of the object outside of the ROI blocked by the collimation.
The above described embodiment reduces an amount of x-ray dosage a patient receives during a CT imaging scan. X-ray dosage outside of an organ of interest (OOI) is minimized whenever a relatively small organ, such as a heart, is the OOI being scanned.